Access sheaths are commonly used to establish an accessible, minimally invasive passageway into the body to facilitate and expediate the insertion and removal of devices. Once access is established, devices can be passed through the access sheath and to the treatment site with increased speed and minimized patient trauma. For instance, typical kidney stone retrieval procedures require multiple insertions and removals of the stone basket and endoscope as successive stone fragments are captured. The access sheath protects the ureter from sharp points or jagged edges of the stone fragments as they are pulled from the ureter or kidney. The access sheath also provides the physician with an established pathway into the ureter avoiding the need to re-establish that path from the urethera through the bladder and into the ureter for each insertion of the endoscope. Thus the procedure is less traumatic to the patient while being easier and faster for the physician.
Due to the nature of their use, access sheaths need to be flexible to follow patient anatomy, provide a maximized working channel for the physician, and be robust enough to confidently endure manipulation. Prior art access sheaths have been constructed with a thin-walled polymer tube. While this construction allows for a flexible access sheath with a maximized working channel, devices of this type are susceptible to kinking, elongation, and ovalization. Kinking and ovalization may render the access sheath useless since instruments may no longer be able to pass through the access sheath to the target anatomy. Furthermore, kinking may cause trauma to the patient or damage to the instruments being used.
Some known prior art access sheaths, such as U.S. Pat. No. 7,005,026, solve the kinking problem by re-inforcing the wall with a wire or wires. In Applied Medical literature, the access sheath is shown tied in a knot to highlight the catheter's extreme kink resistance. While this makes the sheath more resistant to kinking, elongation and ovalization, it increases the thickness of the sheath wall. The increased wall thickness either reduces the working channel, increases the outside diameter or both.
Reduction of the working channel is undesirable for several reasons. In many procedures multiple instruments are needed to be placed at the target anatomy simultaneously, thus requiring a maximized working channel for their placement. Also, in a kidney stone retrieval procedure reducing the access sheath inner diameter may prohibit the extraction of larger stones that would otherwise be extractable through a larger working channel.
Increasing the outer diameter of the access sheath is also undesirable. As the diameter of the access sheath increases it dilates and distends the adjacent anatomy. For instance, in a urological procedure the access sheath can split the patient's ureter if the access sheath's outer diameter is too large. Similar trauma may be caused when entering other patient vasculature.
Another problem with kink resistant, reinforced walls is that they over-bend in the bladder when being pushed up the ureter for the initial placement or when repositioning in the middle of the procedure. This tendency to over-flex and loop into the bladder is common. The bladder is a big open space that does not provide any side support for the access sheath. Once it over bends in the bladder the tip can not be pushed into the ureter. A similar effect can be seen by pushing a straightened finger directly against a wall. It is easiest to push (transmit force to the wall) with a straight finger (0° bend) or a moderately bent finger (up to 90° bend). At 180° bend it is very difficult to place force on the wall.
Whereas manufacturers laud the ability of their access sheaths to bend 360°, it can be seen in the above text that what is needed is an access sheath which is flexible enough to accommodate anatomical bends while being pushable. In addition, an access sheath should accomplish this while maximizing the working channel.